This invention relates to a detector for colored registration marks on contrasting colored backgrounds, and more particularly to such a detector which self-adjusts to the degree of darkness or lightness of the background for detecting light or dark marks thereon respectively.
Present sensors for discriminating between a colored mark on a colored background utilize incandescent lamp light sources which require high power, have limited life and are not optimumly suited for pulsed light detection systems because of the particular afterglow or light response limitations associated therewith. Moreover, such systems must be manually adjusted to calibrate them for each new background color and shade relative to the mark color and shade. Relatively high power consumption levels occur in the incandescent lamp light sources and detector output signals in prior art detectors have low signal to noise ratios for relatively dark contrasting marks and backgrounds. Output signal instability exists due to instability at the edges of the detector threshold.
It is clear that a low power, high sensitivity, self-adjusting mark detector is desirable for a variety of applications, where either dark shade marks on a light shade background or light shade marks on a dark shade background are to be detected.